Researchers from North Carolina State University have described a method for achieving desirable optical properties by stretching shape-memory polymers embedded with gold nanoparticle clusters so as to alter their plasmon-coupling capabilities. The method supports an all-optical thermal history sensor that could be applied to either an object or an environment.

Upon stretching the shape-memory polymer film that the researchers used past its original length by 140%, they were able to determine the highest temperature to which the polymer is exposed (up to 248 °F/120 °C) by measuring how much it has shrunk back to its original size. From the plasmon-coupled nanoparticles present in the system, the researchers showed their ability to measure this change indirectly, exclusively by looking at the optical properties of the material.

Joe Tracy, corresponding author of a paper on the work and a professor of material science and engineering, said that fact means the team was able to rely on light to see just how hot the material got. “An important application of thermal-history sensors is assuring the quality or safety of shipping or storing materials that are sensitive to significant changes in heat,” he said.

When the gold nanosphere-embedded stretchable polymer is heated and stretched and then cooled back to room temperature, the material will hold its adopted shape indefinitely. However, once reheated back to 248 °F, the material returns to its original shape. The gold nanospheres form clusters in the polymer, in which their surface plasmon resonances couple; if the gold nanospheres perfectly dispersed in the polymer, their surface plasmon resonances would remain uncoupled.

Depending on their proximity to one another in the polymer, the optical properties of the plasmon-coupled nanoparticles shift. Those properties change when the polymer shape is stretched.

“When assessing the peak wavelength of light absorbed by the material, there are significant differences depending on whether the light is polarized parallel or perpendicular to the stretching direction,” Tracy said. “For light polarized parallel to the direction of stretching, the further you have stretched the material, the further the light absorbed shifts to the red. For light polarized perpendicular to the stretching direction, there is a blueshift.”

The researchers then concluded that the way in which their polymer recovered its original shape was predictable.

Beyond the empirical development of a thermal history sensor concept, the researchers said they used computational modeling to enhance their understanding of the gold nanosphere clusters and how they changed during stretching. The strength of plasmon coupling is related to the spacings between the nanospheres. This is called a “plasmon ruler.”

The ability to use their optical properties to accurately estimate the distance between plasmon-coupled nanoparticles is informative in designing future polymer nanocomposites based on plasmon-coupled nanoparticles, said Amy Oldenburg, co-author of the paper and a professor of physics at the University of North Carolina.

The research was done with support from the National Science Foundation, the National Institutes of Health, and the Alexander von Humboldt Foundation.

The research was published in ACS Applied Nano Materials (www.doi.org/10.1021/acsanm.1c00309).